Sexual Selection: Definition & Forms

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Why are you attracted to someone? One reason may be due to the forces of sexual selection. This lesson will explain sexual selection and will investigate types of sexual selection.

Sexual Selection

What attracts you to the opposite sex? Do you like men with facial hair and large muscles? Or women with curvy figures and thick, silky hair? Have you ever wondered why you are attracted to a person? What if I were to tell you it had something, in part, to do with sexual selection, or when certain members within a species have an advantage over other members when it comes to mating.

But why a male with facial hair and large muscles or a female with thick, silky hair and a curvy body? Scientists have studied sexual selection in humans and there are many different schools of thought. One is that large muscles and the ability to grow facial hair indicate that the man is healthy and likely has good genes. Females that mate with men who have good genetic material are likely to have offspring that can survive and reproduce.

The same theory suggests that curvy women, with a specific hip to waist ratio, have enough body fat to carry and birth a child. Women are also likely to be healthy with good genes if they can grow thick, silky hair. So you might not realize you are choosing a mate based on his or her ability to contribute good genes to your offspring, but over thousands of years this process of sexual selection has acted as a type of natural selection. In other words, people that have certain characteristics are likely to reproduce, whereas those that do not have those characteristics may not reproduce.

Let's take a moment to explore the different types of sexual selection.

Intersexual Selection and Intrasexual Selection

Before we delve into the types of sexual selection, it's worth taking a moment to differentiate between inter and intra. Inter means between groups. For example an interstate highway goes between the states. Or the Internet is a network that goes between computers. Contrast this with intra, which means within groups. An intrastate highway is a highway that travels inside a state's boundaries. Or intramural sports are sports that are played within your college, but not against other colleges.

Okay, so let's start with intersexual selection, which is sexual selection that is between two sexes. For example, consider a male peacock. Their giant, brightly colored tails make avoiding predators difficult, so what's the advantage of having such tails? Well, females choose male peacocks with brightly colored tails, so even though it's risky to be parading around predators with a bright tail, your chances of mating (and reproducing) are increased and thus this trait gets passed down. The peacock is an example of sexual selection between the two sexes, or intersexual selection. Just like a muscular, bearded human, it's assumed that female peacocks choose brightly colored male peacocks because in order to produce such a large, colorful tail the male peacock must have good genes. These good genes, in turn, will contribute to the success of the offspring.

Female peacocks choosing males with brightly colored, large tails is an example of Intersexual Selection

Contrast this with intrasexual selection, which is sexual selection within a sex. An example here is the competition seen in male primates. In many primates, a male will attempt to keep other males away from females so he can be their primary mate. Large, aggressive male primates with large canine teeth are usually the ones that are able to mate and pass along their genetic material. So here it is not selection from a different sex, it's selection within the same sex. Again, this ensures the female gets the best genetic material from the male because in order for a male to grow large canines and defeat other males, he must be healthy and carry good genetic material.

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